Photovoltaic system performance has come under closer and closer scrutiny in recent years as alternative energy sources have become more important and more common, and as grid-connected systems have become more prevalent. One of the critical challenges in assessing the performance of a large number of deployed systems is the high cost of conventional tools necessary to evaluate system performance.
The present invention is directed toward providing an apparatus and a method for assessing the performance of photovoltaic systems. In particular, the present invention is directed to providing an apparatus and a method which can satisfy the need for a simple, accurate and less expensive way to evaluate the performance of any particular photovoltaic system and continuously estimate the theoretical power (referred to herein as the “maximum power level”) that should be produced by the photovoltaic system at any given instant under the then-prevailing conditions. If the instantaneous levels of power being produced by the photovoltaic system are not meeting such estimated power levels, it would then be apparent that there may be one or more problems with the photovoltaic system, which problem(s) is/are adversely affecting the power being produced by the photovoltaic system. Accordingly, steps could immediately be undertaken toward diagnosing and then correcting the problem or problems.
Relatively expensive pyranometers have been used in the past for sensing irradiance of light striking photovoltaic arrays. Irradiance is, however, just one of the factors which affect the maximum power level of a photovoltaic system. In particular, the temperature of the photovoltaic cells in the photovoltaic system also affects the maximum power level. In addition, the spectral response of the photovoltaic cell or cells, and the spectrum of sunlight at any given instant also affect the maximum power level. Use of a pyranometer alone does not account for temperature effects. If a pyranometer were combined in a system with one or more thermocouples or thermistors for sensing the temperature at one or more locations on the photovoltaic array, an expensive data logger would be needed for reading and interpreting such data.
Another factor which complicates estimation of the maximum power level of a photovoltaic system is the fact that irradiance, temperature and spectrum of sunlight typically change over the course of each day, and in some cases, one or more of these properties change almost continuously for a period of time. In addition, estimation of the maximum power level is further complicated by the fact that irradiance and temperature each affect the maximum power level non-linearly, as discussed below. For example, FIG. 1 is a graph showing the character of different current-voltage curves (each such curve is referred to herein as an “IV curve”) for a particular representative photovoltaic cell at two different irradiance levels, with temperature and spectrum of sunlight being constant. As can be seen from FIG. 1, even at constant temperature and spectrum of sunlight, a change in irradiance causes a photovoltaic array to operate according to a different IV curve.
Conventional calculations of predicted array maximum power typically use at least (1) one or more silicon solar cells to measure irradiance, (2) one or more thermisters or thermocouples to measure an array's operating temperature, and (3) one or more data loggers equipped to read the temperature data.
The present invention is directed to a simple, accurate and less expensive apparatus and method that can be used to continuously estimate the maximum power level of a photovoltaic system while taking into account the instantaneous variations in plane of array irradiance and temperature.